The present invention relates to an image processing apparatus for performing image processing for an input color image in a digital color copying machine that reads a color image in an original document and forms a reproduced image thereof.
In a color copying machine, a color image in an original document is color-separated and read by a color line CCD sensor or the like. Then, the read image is converted into a color material signal, such as that for a color toner or a blacking, and printed. This conversion is referred to herein as color conversion.
In the color conversion, by performing a predetermined computation for color separation signals R (red), G (green) and B (blue) of the color image, these color separation signals are converted into color material signals C (cyan), M (magenta) and Y (yellow).
In order to perform an automatic background removal of C, M and Y signals, there has been conventionally used a method in which a background upper limit threshold is detected from a histogram of each color and a density conversion curve is defined for each color.
In accordance with such a method, however, as the processing is independently performed for each color, there arises a problem in that a color that is assumed as the background and that has decreased density and a color with its density not being decreased exist at the same time, and a hue is improperly varied.
Further, image processing having higher resolution than that of gray signal value processing, blacking processing, blacking substitution processing and the like is required.